1. Field of the Invention
The present invention relates to an apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Apparatuses are used in so-called coating systems for providing a traveling material web, formed for example of paper, cardboard or a textile material, on one or both sides with one or several layers of the coating medium, for example color, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicator system directly onto the surface of the traveling fiber material web. The web is carried during application on a rotating backing surface, for example an endless belt or a backing roll. In the indirect application of the medium, the liquid or pasty coating medium is first applied onto a substrate, for example the surface of a backing roll configured as an applicator roll. The coating medium is then transferred from the substrate to the fiber material web in a nip through which the fiber material web passes.
Known from German Utility Model No. 295 20 686.1, assigned to the assignee of the present invention, is an applicator for direct application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard. The applicator includes a metering system which is movable from a standby and/or maintenance position to an operating position. The metering system has a metering slot configured as a nozzle and formed between an approach-side lip and a departure-side lip. The applicator also includes a traveling countersurface opposing the nozzle and having the form of an applicator roll or backing roll on which the coating medium issuing out of the metering slot is to be applied. Of the two lips forming the metering slot, the one disposed on the side of the metering slot which in the indirect application of the medium the applicator roll rotates toward, or which in the direct application of the medium the fiber material web runs toward, is called the approach-side lip. Accordingly, the second lip, located on the side of the metering slot from which the applicator roll or fiber material web departs the applicator, is called the departure-side lip. In one embodiment of this applicator, a doctor element, for instance a doctor blade or roll doctor, serves as an applicating doctor and is arranged on the free end of the departure-side lip. The doctor blade or roll doctor scrapes the coating medium, which is applied at surplus, down to a specific coating thickness or a specific profile. In another configuration variant of this applicator, the exit of the metering slot is followed by a concave deflector plate which borders on the approach-side lip and deflects the coating medium issuing out of the metering slot. This metering system is usually followed by a separate doctor element thinning out the coating medium applied on the countersurface.
Known from U.S. Pat. No. 5,436,030, moreover, is an applicator including a concavely curved rigid deflector surface for the liquid or pasty coating medium. The deflector surface borders on the metering slot configured as a nozzle.
In conventional applicators of the type described above, it is necessary to first apply the coating medium at surplus on the traveling countersurface and subsequently scrape it down to a required profile by means of a (mostly separate) doctor element. Resulting therefrom is the use of a relatively large amount of coating medium as well as an appreciable equipment expense along with accordingly increased manufacturing expense and energy demand of the applicator. The increase in energy demand is specifically through higher pump capacity requirements, which, in turn, expresses itself accordingly in the manufacturing and operating costs of these applicators. Besides, backup or backflow phenomena having a negative effect on the coating quality occur at the line of impingement of the coating medium jet on the traveling countersurface, or directly before the doctor element. Furthermore, conventional applicators involve undesirable boundary layer phenomena. For instance, the coating medium makes, on account of the previously illustrated applicator design, direct contact with the boundary layer of air entrained by the traveling countersurface. This leads to air inclusions in the coating medium and a quality reduction of the produced coating. Also, the bent or angled deflector plates or the specially configured nozzle lips with molded, rigid deflection surfaces, such as used in prior applicators, are relatively expensive to fabricate and, consequently, high in cost.
The present invention provides an improved applicator such that the disadvantages associated with the prior art, notably the described boundary layer phenomena, will be avoided easily and to maximum extent. The present invention also provides an improved applicator for the production of a high-quality coating.